Prestressed concrete joist and slab construction



Feb. 7, 1967 C. A. PRATT IRESTRESSED CONCRETE JOIST AND SLAB CONSTRUCTION Filed Jan. 7, 1964 2 Sheets-Sheet l Q 5 W R A o R W a Q C M M R M M a w N. mlN rnm Q +m 3 H a III III III |I WWW A, L l: T: 1a T F F i r fl w E E um l m E m T m mm c. A. PRATT PRESTRESSED CONCRETE JOIST AND SLAB CONSTRUCTION Filed Jan. 7, 1964 2 Sheets-Sheet 2 k mN ffN .M HH LHN INVENTOR. I CAHRENCE 4. P247? Quw United States Patent 3,302,348 PRES'IRESSED CONCRETE JOIST AND SLAB CONSTRUCTION Clarence A. Pratt, St. Paul, Minn, assignor to Perl-Tile Company, Minneapolis, Minn., a corporation of Minnesota Filed Jan. '7, 1964, Ser. No. 336,250 3 Claims. (Cl. 52-223) This invention relates to an improvement in prestressed concrete joist and overlying slab construction.

It is desirable to have such a construction which is adapted to withstand intense heat for prolonged periods of time.

It is further desirable and an object of this invention to provide a unitary structure adapted to meet and successfully undergo certain specified fire endurance tests in order to have certain recognition as a specified fire-safe construction.

As a more specific object of this invention, the applicant provides a very simple construction resulting in a unitary structure which has successfully withstood recognized fire endurance tests and has been approved as qualified to carry a label indicating successful passage of such tests. Specific reference is had to approval received of the Underwriters Laboratories, Inc. in having passed its well recognized and accepted fire endurance tests and in having become qualified for their fire label service. Such a construction is desirable in that it materially reduces fire insurance costs.

These and other objects and advantages of the invention will be fully set forth in the following description made in connection with the accompanying drawings in which like reference characters refer to similar parts throughout the several views and in which:

FIG. 1 is a fragmentary view in vertical cross section showing the installation of an element of applicants construction in a mold;

FIG. 2 is a view in elevation on a reduced scale with some portions thereof shown in dotted line and a portion thereof having a fragmentary overlying slot shown in vertical section;

FIG. 3 is a view in vertical section taken on line 3-3 of FIG. 2 as indicated;

FIG. 4 is a view similar to that in FIG. 3 free of a mold;

FIG. 5 is a view in vertical section taken on line 5--5 of FIG. 2 as indicated;

FIG. 6 and 7 are views of a detail of applicants construction shown in front and side elevation; and

FIGS. 8 and 9 are views of another detail of applicants construction in front and side elevation.

The applicant provides a structure which has passed recognized fire endurance tests. More specifically it has passed the fire endurance tests of the Underwriters Laboratories, Inc. and is qualified to bear a label to indicate such approval.

The applicants concrete is formed in a manner customary for prestressed concrete joist construction and embodies the use of commonly used prestressed strands or wires. The applicant provides a joist of relatively small cross section and in various lengths as up to forty feet. In tests conducted to deter-mine fire endurance of a joist of relatively small cross section, there appears to be a common tendency for such a joist to crack and split in the plane of or along the axes of the prestressed wires. This appears to be a plane of weakness. This tendency appears to be present whether the prestressed wires are vertically aligned centrally of a joist in a single plane or whether they are disposed in an offset or staggered relation relative to one another. The applicant provides an improvement in joist construction which has overcome this tendency and which successfully passes a fire endurance test.

With attention being directed to joists of relatively small cross section, particular interest is had in joists 0n the order of 16 inches in height with a 6 inch width at the top tapering to a 4 inch width at the bottom. It is obvious that there is relatively small width in the joist to employ whatever additional element is necessary to prevent the longitudinal splitting thereof along the axes of the prestressed wires.

The appplicant in the embodiment of his structure here disclosed adds but a single element to the joist construction to have it successfully withstand the axial stresses of the prestressed wires and be free of cracking or splitting in being subjected to a fire endurance test. In connection therewith, applicant provides a unitary joist and slab con struction.

With reference to the drawings, a mold 10 is shown in end elevation in FIG. 1 disposed in what is here indicated as being a concrete supporting structure 12 which form of support for a mold is common in the art.

For purpose of illustration herein, said mold is designed to provide a joist 18 having a cross section as above indicated. Shown in position running through the mold are the prestressed strands or wires 13, 14 and 15. These strands may vary in number and in position but are here shown in vertical alignment. The strands will be prestressed by means commonly employed in the art and not here shown. It is obvious that there is but a very small space within the mold to position any element of structure.

With the prestressed strands positioned, the applicant introduces an element which when embodied within the construction of the joist to be formed successfully restrains longitudinal cracking or splitting of the joist as along the plane of the strands.

The element used is the member 17 formed of rod-like material having a spring-like characteristic and preferably being formed of stock such as one-half inch in diameter on the order of a No. 4 reinforcing bar. Said member 17 will preferably be formed to have a width such that it will be spaced inwardly uniformly of the sides of the mold as on the order of three-fourths of an inch positioned therein cross-wise, as indicated in FIG. 3. Hence said member will take on an inverted substantially triangular form.

Said member 17 comprises an upper web portion 19 having extending downwardly thereof somewhat inwardly inclined arm portions 20 and 21. Said arm portions have free end portions 22 and 23 inclined inwardly in a direction to intersect or cross over one another. The portion 23 is partially downwardly reversely bent to form a hook portion 24. The portion 22 has a free end depending portion 25 reversely formed or bent to be U-shaped and formed in a plane normal to the plane of said hook portion and of the body portion generally of said member 17. Said hook portion is arranged to pass through said U-shaped portion when said arm portions are urged together to intersect said portions 22 and 23. Said arm portions are spaced apart sufiiciently to be under considerable tension when urged together, as above indicated. Further, normally said portions 23 and 24 are spaced apart as indicated in FIG. 8.

To be placed in operating position, said member 17 is lowered into the mold 10 by hand by being grasped at the upper portion thereof and by having the spaced free ends thereof pass by the upper strands or wires 13 and 14. With the free end portions 24 and 25 positioned above the level of the strand 15, the arms 20 and 21 are urged toward one another to intersect the portions 22 a and 23 and to move the hook portion 24 through the U-shaped portion 25. Member 17 is then lowered to have said hook and U-shaped portions at either side of said strand 15. Said arms are released to spring in the direction of their normal positions and in so doing snap the hook and U-shaped portion together with said strand being engaged and locked therebetween. Said U- shaped portions is provided with sufiicient width to position and hold the member 17 in a plane at right angle to the axis of said strand 15. Members 17 will be spaced longitudinally of said mold on the order of intervals of 12 inches.

The upper or web portion 19 of said member 17 may be suitably supported by wires 28 or by other suitable means as may be expedient. Such support is temporary and is required only while the concrete is poured to assure that the vertical alignment of the members 19 will not be disturbed by the pouring.

Said arms 20 and 21 normally will be spaced apart to have said portions 23 and 24 in spaced relation as indicated in FIG. 8, and to come under a certain degree of tension when urged together sufiiciently to have said portions 22 and 23 in an intersecting relation as indicated in FIG. 1.

It is appreciated that stirrups per se have been used in precast concrete joists and structures. These might well be regarded as being generally on the order of applicants member 17. However in the history of the art it appears that stirrups are commonly used to position reinforcing rods or strands as by being welded to them.

The applicants member 17 in no way functions to support or position the prestressed strands as these are self-positioned. The applicants member 17 has no required connective relationship with such strands. It has been specifically developed to provide a member which could be readily positioned in a relatively narrow mold to be embodied within the joist formed therein. The specific holding portion of the member 17 has been particularly developed to permit a ready insertion of the member into a mold with a clipping or snapping action which will permit the member 17 to readily engage and be supported by the lowest strand in an upstanding position, and the holding portion has been particularly designed to cause the body portion of the member 17 to be held in a position crosswise of the mold. It is to be noted that the space within the mold is limited and relatively inaccessible. Hence the applicant provides his ingenious and elfective member 30 which is readily positioned as described.

It is believed that the presence of the member 17 disposed transversely within a prestressed joist construction restrains the axial load of the prestressed strands to prevent a separation or splitting along the plane of the strands when the joist is subjected to intense heat, such as in a fire endurance test. It is known that with the presence of the member 17, splitting of the joist was avoided in the testing which was conducted. In the absence of the member 17, cracking or splitting of the joist was present as a result of such test.

In connection with the joists formed, an upstanding shear restraining or anchor member 30 is used to tie together with the joist the overlying slabs 32 to form a unitary structure. Said slabs are commonly on the order of 2 x 4 feet in size providing for a spacing of the joists of 4 feet on center. Said member 30 is hereafter referred to as an anchor member and is shown in the form of a rectangular plate of suitable sheet material, such as of steel, and may be on the order of 2 X 6 inches in size.

Said member 30 will be suitably positioned by hand after the concrete is poured into the mold and has set sufliciently to support said member. Thus said member 30 then becomes integral with the joist. Said member 30 has a lower aperture 30a of substantial size to permit a web of concrete to extend therethrough to hold the same. A smaller upper aperture 30b is provided which is a depth marker indicating the depth to which the member is to be inserted into the concrete and said member has a partially split upper end portion 300 projecting from the joist having finger portions 30d and 30e which are angled oppositely one another to form a Y, as viewed in FIG. 4.

In the composite structure or assembly, as indicated in FIG. 5,- slabs 32 are indicated in position with said member 30 in operating position relative thereto. The adjacent ends of the slabs will be beveled. The area between said adjacent ends and about the projecting portion of said anchor member 30 will be filled with a grout 35. Thus the joist and overlying slab tied together by the member 30 become an integral assembly or structure. The angled fingers of the member 30 pre- Vent any tendency of uplifting on the part of the ends of the slabs which might otherwise result from the effects of an atmospheric condition, such as a heavy wind storm. Further, said anchor members provide lateral stiflness to the joists to overcome the tendency for transverse buckling and in the composite structure there is provided great lateral stability.

Thus I have provided a simple and effective improvement in prestressed concrete joist construction and assembly which based on recognized tests conducted has successfully undergone fire endurance testing.

It will of course be understood that various changes may be made in the form, details, arrangement and proportions of the parts, without departing from the scope of applicants invention which, generally stated, consists in a construction capable of carrying out the objects above set forth, in the parts and combinations of parts disclosed and defined in the appended claims.

What is claimed is:

1. A joist construction having in combination:

a concrete joist,

a plurality of prestressed strands running longitudinally through said joist,

a plurality of transversely disposed members spaced longitudinally through said joist,

said members being formed of resilient rod-like material comprising a web portion, and depending arm portions,

one of said arm portions having a reversely curved free end portion forming an outwardly facing hook portion, and

said other arm portion having its free end portion reversely curved to form a U-shaped portion disposed in a plane normal to the plane of said member and of said hook portion whereby when said arm portions are urged toward one another said hook portion passes through said U-shaped portion to face the same to engage one of said strands therebetween, said U-shaped portion having sufiicient width in engaging said strand to position said member in a plane normal to the axis of said strand.

2. A joist construction having in combination:

a concrete joist,

a plurality of prestressed strands running longitudinally through said joist,

a plurality of transversely disposed members enclosing said strands spaced longitudinally through said beam,

each of said members formed of resilient rod-like material comprising a web portion and spaced extended arm portions,

the free end portion of one of said arm portions being curved to form an outwardly facing hook portion,

the free end portion of said other arm portion being reversely curved to form a U-shaped portion disposed in a plane normal to the plane of said member and of said hook portion whereby when said arm portions are urged toward one another said hook portion passes through said U-shaped portion to face the same to engage one of said strands therebetween,

said U-shaped portion having suflicient width upon engaging said strand to position said member to be in a plane normal to the axis of said strand, and

said member having a configuration such that when in operating position engaging said strand said arm portions will be uniformly spaced from the adjacent wall portions of said joist.

3. A composite structure comprising:

a concrete joist,

a plurality of prestressed strands running longitudinally through said joist,

a plurality of members elongated loop-like in form of rod-like material disposed transversely within said joist in longitudinally spaced relation enclosing said strands,

each of said members having free lower end portions arranged and constructed for locking engagement with said strands, said lower free end portions being adapted to position said members in planes normal to the axis of said strand,

a plurality of anchor members integral With said joist spaced longitudinally thereof and projecting upwardly thereof,

slabs supported on said joist at either side of said anchor member,

means engaging and tying said anchor members and said slabs into a unitary structure with said joist to withstand transverse load for lateral stability,

said lower end portions being inclined to intersect one another in the plane of said member, one of said end portions forming a hook-like portion, said other end portion forming a depending U-shaped portion in a plane normal to the plane of said member and of said hook-like portion whereas said hook-like portion passes freely through said U-shaped portion engaging said strand therebetween, and

said U-shaped portion being of sufiicient width in engagement with said strand to cause said member to be disposed in a plane normal to the axis of said strand.

References Cited by the Examiner UNITED STATES PATENTS 485,758 11/1892 Delug 24261 1,265,139 4/1918 Tittle 24-261 1,722,159 7/1929 Sexton 24131 1,735,787 11/1929 Peterson 24-131 2,137,009 11/1938 Stromberg 52-434 X 2,435,998 2/1948 Cueni 52223 FOREIGN PATENTS 943,027 1948 France.

589,649 1959 Italy.

120,458 1927 Switzerland.

FRANK L. ABBOTT, Primary Examiner.

JOHN E. MURTAGH, Examiner. 

3. A COMPOSITE STRUCTURE COMPRISING: A CONCRETE JOIST, A PLURALITY OF PRESTRESSED STRANDS RUNNING LONGITUDINALLY THROUGH SAID JOIST, A PLURALITY OF MEMBERS ELONGATED LOOP-LIKE IN FORM OF ROD-LIKE MATERIAL DISPOSED TRANSVERSELY WITHIN SAID JOIST IN LONGITUDINALLY SPACED RELATION ENCLOSING SAID STRANDS, EACH OF SAID MEMBERS HAVING FREE LOWER END PORTIONS ARRANGED AND CONSTRUCTED FOR LOCKING ENGAGEMENT WITH SAID STRANDS, SAID LOWER FREE END PORTIONS BEING ADAPTED TO POSITON SAID MEMBERS IN PLANES NORMAL TO THE AXIS OF SAID STRAND, A PLURALITY OF ANCHOR MEMBERS INTEGRAL WITH SAID JOIST SPACED LONGITUDINALLY THEREOF AND PROJECTING UPWARDLY THEREOF, SLABS SUPPORTED ON SAID JOIST AT EITHER SIDE OF SAID ANCHOR MEMBER, MEANS ENGAGING AND TYING SAID ANCHOR MEMBERS AND SAID SLABS INTO A UNITARY STRUCTURE WITH SAID JOIST TO WITHSTAND TRANSVERSE LOAD FOR LATERAL STABILITY, SAID LOWER END PORTIONS BEING INCLINED TO INTERSECT ONE 